We discuss climate models a lot, and from the comments here and in other forums it's clear that there remains a great deal of confusion about what climate models do and how their results should be interpreted. This post is designed to be a FAQ for climate model questions - of which a few are already given. If you have comments or other questions, ask them as concisely as possible in the comment section and if they are of enough interest, we'll add them to the post so that we can have a resource for future discussions. (We would ask that you please focus on real questions that have real answers and, as always, avoid rhetorical excesses).
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With the remarkable admission by Alan Greenspan that his “theory” of economics had a fundamental flaw,
I’ve been musing about all those emails I get from people claiming to have found a “fundamental flaw” in the “greenhouse theory”. The letters range from amusing claims that we are overlooking changes in the magnetic field, to tales about how the “weight” of carbon dioxide keeps it “near the ground”. If the writer sounds serious, then I treat them seriously, and do my best to provide a helpful reply. Often, though, I find myself in a pointless debate of the most basic, well-established physical principles. I generally cut off the discussion at this point, because I simply don’t have the time. This can result in a hostile response accusing me of “having an agenda”. Most would call me naïve for bothering to respond in the first place.

But it is possible, after all, that somewhere in that barrage of letters lies a brilliant idea that ought to be heard, and could change the course of scientific history. How to tell the difference? Well, there is a story that we tell in our family that might provide some perspective on this.

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I had the opportunity to attend a three-day meeting of the Nature Conservancy last week in Vancouver. I was there with my RealClimate hat on, to offer ideas and insight on blogging in particular, and public communication of science in general.

While at the conference, I had a rare chance to see some of the inner workings of one of the world's largest and most successful environmental organizations (this was an invite-only conference, and only a handful of us were other than Nature Conservancy staff and trustees). The Nature Conservancy has a reputation of being very non-partisan, and this was abundantly evident at the conference: There were representatives present from the Shell Oil Company, from the Christian Coalition, from Environmental Defense. This broad level of buy-in of Nature Conservancy goals is perhaps not surprising, given that the main thing this organization is known for is its method of protecting land: buying it. Neither free-market boosters (if there are there any remaining) nor lefty environmentalists have any trouble with this.

What was news to me, though, was the extent to which the Nature Conservancy is also working towards influencing policy on climate change. Getting serious about climate policy is no longer a partisan issue in the U.S.: both John McCain and Barak Obama are on record for supporting cap and trade carbon markets. But one might well ask what climate policy has to do with the buy-the-land-to-protect it method of the Nature Conservancy. As I learned at the conference, there are two rationales.

First, the Nature Conservancy has a strongly science-based policy for making land-purchasing decisions. They take into account things like the minimum viable ecosystem size in determining which acquisitions will actually have lasting impact. Trouble is, for many areas, the conditions those decisions are based on may change. Areas near sea level are an obvious example. But so are the more than 10,000 acres of native tallgrass prairie that they have protected in Kansas. How much will that ecosystem change with the projected changes in precipitation in this region? Obviously, the Nature Conservancy is taking into account such projections, as best they can. But they have also decided that the risks of climate change to the world's ecosystems are too large to simply adapt to: hence their interest in helping to push governments to enact policies that will help mitigate it.

Second, it turns out that the Nature Conservancy's mission is — and has always been — much broader than is widely recognized. As they note on their mission statement page they can't possibly buy all the places they want to protect. To achieve their mission — "to preserve the plants, animals and natural communities that represent the diversity of life on Earth by protecting the lands and waters they need to survive" — they will clearly have to do something much bigger. The buzzword here is sustainability, and the nature Conservancy is now launching what they call their Campaign for a Sustainable Planet. This means a serious focus not just on direct ecosystem protection but also on sustainable development. International sustainable development goals, of course, are impossible to separate from international energy policy. And one cannot today talk about energy policy without talking about climate policy. It is therefore quite logical for the Nature Conservancy to be drawn into weighing-in on climate policy.

Now, what does this have to do with RealClimate? Well, the Nature Conservancy is interested in expanding their web-based communication toolbox. They already have a great deal of information on their web page: see for example their article on climate change adaptation, here; but they'd like to do more. And in particular, they are interested in getting more information out their from their scientists. As I already pointed out, The Nature Conservancy — which has over 700 full time scientists working for it — prides itself on being strongly science-based. So do we here at RealClimate (read our welcome page). Unlike us, whose day jobs are to produce and broadly disseminate scientific results (through teaching and publication), Nature Conservancy scientists don't necessarily publish their work. There is no doubt a lot of really interesting and important work being done that doesn't get out to many other scientists, let alone the general public. A blog, or something like it, could provide those scientists with a place to talk about their work. Hence my attendance at the meeting, to offer a bit of advice and perspective.*

Here at RealClimate, we look forward to seeing where Nature Conservancy goes with this, and wish them best success in their efforts. [Watch this space for an announcement, if and when they launch something.] For my part, I'll be especially enthusiastic if the Nature Conservancy doesn't limit itself to talking about its various projects around the world (interesting as those may be). What I'd really like to see is a site that provides their perspective on some of the more difficult — but really important — questions in the area of climate change impacts. How much danger are polar bears in, really? How will agriculture in Asian monsoon regions be affected? What are the broader effects of ocean acidification (beyond the immediate impact on coral reefs)? The Nature Conservancy won't be the last word on this — any more than RealClimate is. But their perspective, from field scientists "on the ground", could prove extremely valuable.

One additional thought. In several of the sessions I attended at the Nature Conservancy meeting, reference was made to the need to stabilize global temperature rise at no more than 2°C, and correspondingly to stabilize CO₂ levels at no greater than 450 ppm (strictly speaking, this should be 450 ppm CO₂-radiative-equivalent; there is a big difference, and it is often neglected). In each case, reference was made to the IPCC reports as the source of these numbers. Yet these numbers really don't derive from the IPCC, which (rightly) shied away from being policy prescriptive. Rather, they have their origin in a small number of documents, notably the 1995 report of the German Advisory Council on Global Change, papers related to the Exeter conference on "avoiding dangerous climate change" and in European Union Council decisions (see the 1996 and 2006 Presidency Conclusions here). Clearly, the 2°C/450 ppm numbers have completely permeated the policy-advocacy realm. Yet while they are arguably derivable from the IPCC reports, it is actually not clear to what extent the larger scientific community really believes these are the right numbers. There simply has not been a process to evaluate this that compares in depth and breadth with the IPCC. A new and much more comprehensive analysis, by a much greater group of scientists, would be valuable at this juncture. Scientists are fond of saying that they cannot summarize their projections with a small handful of simple numbers, but simple numbers are what are being discussed in policy circles. If the "right" numbers are really so low as Jim Hansen believes (see our post on this, here), then the Nature Conservancy has an even more difficult task ahead.

*[For those interested in slogging through it, there's a video of our session, here. It's not just about me. There is also some really interesting stuff from Nature Conservancy staffer Jonathon Colman and from the folks who started the conservation clearinghouse and collaboration websites www.conservationyellowpages.org and wiserearth.org, as well as a demonstration (not altogether successful) of the emerging virtual reality conferencing technology which (when it works) might help all of us travel to conferences less often. A word of advice: skip all this and instead take a look at the Keynote lectures, by far better orators than me: Mark Tercek, Nature Conservancy CEO, Jerome Ringo, Apollo Alliance President, and CARE C.O.O. Steve Hollingworth

Many readers will remember our critique of a paper by Douglass et al on tropical tropospheric temperature trends late last year, and the discussion of the ongoing revisions to the observational datasets. Some will recall that the Douglass et al paper was trumpeted around the blogosphere as the definitive proof that models had it all wrong.

At the time, our criticism was itself criticised because our counterpoints had not been submitted to a peer-reviewed journal. However, this was a little unfair (and possibly a little disingenuous) because a group of us had in fact submitted a much better argued paper making the same principal points. Of course, the peer-review process takes much longer than writing a blog post and so it has taken until today to appear on the journal website.
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The theme of this year's annual meeting of the European Meteorological Society (EMS) [European Congress on Applied Climatology (ECAC)] was adaptation to climate change. So what's more appropriate then, than hosting the meeting in Amsterdam - on a building site?

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Guest commentary from Mauri Pelto

Changes occurring in marine terminating outlet glaciers of the Greenland Ice Sheet and ice shelves fringing the Antarctic Peninsula have altered our sense of the possible rate of response of large ice sheet-ice shelf systems. There is a shared mechanism at work that has emerged from the detailed observations of a number of researchers, that is the key to the onset and progression of the ice retreat. This mechanism is shared despite the vastly different nature of the environments of Jakobshavns Isbrae, Wilkins Ice Shelf and the Petermann Glacier.
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Here at RealClimate we understandably have an intense interest in the positions of the Presidential and Vice-Presidential candidates regarding global warming and carbon emissions. What the stance bodes for future action on climate change is consequential in itself, but beyond that the ability to use sound science in this case serves as a bellweather for the candidates' whole approach to science. Whatever else you can say about the candidates, it has been encouraging that both John McCain and Barack Obama favor mandatory action to reduce US carbon emissions.

But, enter Gov. Sarah Palin, McCain's pick for VP. Palin's position on global warming has been stated quite clearly in this recent interview with the publication Newsmax , where she says "A changing environment will affect Alaska more than any other state, because of our location. I'm not one though who would attribute it to being man-made." How is this to be reconciled with McCain's position? Do they just agree to differ? What does this bode for future actions if McCain were to win the election, especially in view of the fact that, in a Cheney-esque way, Palin is likely to be put in charge of energy policy? The recent vice-presidential debate sheds some light on the issue. A full transcript of the debate is here.

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Dennis Bray and Hans von Storch have been making surveys of climate scientists for a number of years with the reasonable aim of seeing what the community thinks (about IPCC, climate change, attribution etc). They have unfortunately not always been as successful as one might like - problems have ranged from deciding who is qualified to respond; questions that were not specific enough or that could be interpreted in very different ways; to losing control of who answered the questionnaire (one time the password and website were broadcast on a mailing list of climate 'sceptics'). These problems have meant that the results were less useful than they could have been and in fact have occasionally been used to spread disinformation. How these surveys are used obviously plays into how willing scientists are to participate, since if your answers are misinterpreted once, you will be less keen next time. Others have attempted similar surveys, with similar problems.

As people should know, designing truly objective surveys is very tricky. However, if you are after a specific response, it's easy to craft questions that will favour your initial bias. We discussed an egregious example of that from Steven Milloy a while ago. A bigger problem is not overt bias, but more subtle kinds - such as assuming that respondents have exactly the same background as the questioners and know exactly what you are talking about, or simply using questions that don't actually tell you want you really want to know. There are guides available to help in crafting such surveys which outline many of the inadvertent pitfalls.

Well, Bray and von Storch have sent out a new survey.

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What reduces emissions more?
A. Someone swapping their old SUV (which gets 12 miles per gallon) for a hybrid version (18 mpg) or
B. someone upgrading their 25 mpg compact to a new 46 mpg Prius?
(ignore for a minute manufacturing issues or driving habits and assume the miles driven are the same).

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In a recent post about sea level rise, we highlighted a paper by the University of Colorado's Tad Pfeffer and others in which they show that one can rule out more than 2 meters of sea level rise in the next century. While we liked the paper very much, we also complained that Pfeffer and colleagues had created a bit of a straw man, by implying that it had been seriously proposed that Greenland's near term contribution to sea level rise could be much larger than that. In fact (we said), none of us in the climate science community ever took such ideas seriously, even if the popular press thought we did. Tad responds by pointing out that in fact there is published work attributing considerable likelihood to such extreme scenarios, and that there are numerous studies that at the very least strong imply it. He also reminded me that their paper actually rules out a contribution of more than about 50 cm from Greenland, significantly below some other recent published estimates. That makes their work even more important, since there are several publications that definitely consider upwards of one meter (from Greenland alone) by 2100 to be plausible. Pfeffer et al. conclude that that is simply not the case (at least in their informed view). Still, we remind readers that our chief complaint was that Pfeffer et al.'s work was taken by many in the media as a downward revision to sea level rise estimates, whereas in fact most informed estimates had put an upper limit well below that. See our earlier post on the IPCC Sea Level numbers.

A response to RealClimate’s post on our paper about sea level rise

W.T. Pfeffer, J.T. Harper, and S. O’Neel
15 September 2008

We have read with interest – and, we admit, surprise – the RealClimate post concerning our 5 September publication in Science entitled “Kinematic Constraints on 21st Century Sea Level Rise.” The source of our surprise, however, is probably not what the RealClimate authors imagine – we had fully expected a vigorous defense of very high rates of sea level rise (greater than 2 m/century), but not a denial that such rates had ever been hypothesized.

We do not state anywhere in our paper that 2m or more of SLR by 2100 has been published as a peer reviewed and “informed estimate”. We do state that this has been ‘inferred’ and ‘argued’ as a “viable 21st century scenario”. We believe there is value in constraining the upper limits to the role of ice dynamics in future SLR. And, from what we know about historical rates of SLR in conjunction with what ‘we know we don’t know’ about ice dynamics, we believe it is reasonable to ponder very high rates of SLR in the next century. However, we also believe that it is problematic to project such a ‘hypothesis’ as a supported theory without proper testing by the scientific method. The question raised by RC is whether or not this hypothesis has circulated within the scientific community.

In his 2007 paper (Environ. Res. Lett. 2(2007)) Hansen proposes a rate of sea level rise of “5 m this century.” This is hypothetical, but he is confident that it is a “far better estimate than a linear response”. This is accompanied by his statement that he finds it “almost inconceivable that BAU climate change would not yield a sea level change of the order of meters on the century timescale.” The provisional nature of his discussion is irrelevant; it is an explicit statement that 5 m of sea level rise in this century is a possibility he regards as viable, published in the scientific literature by the person who is arguably (and deservedly) the most visible and authoritative climate scientist in the world. No reader of this paper would assume that Hansen didn’t actually mean what he said. Hansen reinforced this idea in other publications and statements, including in his briefing to Congress on 23 June 2008 (“sea level rise of at least two meters is likely this century”). Our analysis specifically tested the likelihood of next-century sea level rise of more than 2 m, and Hansen explicitly hypothesized 5 m of sea level rise in this century.

Hansen has gone on record with specific numbers, but other published studies including the 2006 Overpeck and Otto-Bliesner Science papers left the upper limit open ended, and certainly implied it could be quite high. The fact that this idea was present in the scientific community was confirmed for us by 8 scientific presentations we gave on this topic in the past year (5 in the US, including the Fall 2007 AGU and 3 in Europe). At none of those talks did anyone in the audience question what high forecasts we were referring to. The comments we got back on our work were overwhelmingly positive, and were along the lines that what we had presented was a good next step – both to move past the IPCC’s low sea level forecasts, and as a response to the persistent hypotheses of very high rates of sea level rise that were circulating. Criticisms, where they were voiced, were largely that we were underestimating the power of dynamics and that rates of sea level rise well in excess of 2 m/century might occur in spite of our conclusions.

We agree that the media coverage of our paper (as well as others before it) has undesirable side effects. Wherever we had the opportunity we pressed media writers not to use terms like “exaggerated” or “high sea level forecasts debunked,” and we have consistently stressed that our results indicate a very significant sea level rise and are no justification for any kind of complacency. We have stressed that even our low end scenario of 0.8 m of SLR would have tremendous consequences. However, we stand by our statements that sea level rise at rates of substantially more than 2 m this century were in fact put forward as a likely possibility.

Earlier this summer Andy Revkin published a piece in the New York Times about what he has termed the “Whiplash Effect”: confusion created in the public mind by media coverage of rapidly evolving scientific ideas. There has certainly been some whiplash in this case. However it is others who cracked the whip. We have simply refused to let go of the other end.